A lot will depend upon whether you are doing incremental backups or reverse incrementals. With reverse incrementals, you have 1 .VBK file that gets appended to daily, and a .VRB is created for each previous point in time. So, with 30 day retention, you would have 30 files, and when the 31st file gets created, the oldest .VRB is deleted.
If you use incremental backups, its far more complex. Image if you will, that you do backups on weekdays only (M-F), and you do a synthetic full backup on Friday. Lets say that you want to keep 10 restore points.
1st Monday: Full backup
1st Tuesday: Incremental
1st Wed: Incremental
1st Thurs: Incremental
1st Fri: Incremental first, then sythentic runs, leaving new full backup (and this particular incremental is deleted).
So, after the 1st week: You have 2 full backups (1st monday and 1st friday), with 3 incrementals (.vib's on Tuesday-Wednesday-Thursday). And you have a total of 5 more points.
2nd Monday: Incremental
2nd Tuesday: incremental
2nd Wed: Incremental
2nd Thurs: incremental
2nd Friday: Incremental first, then sythentic runs, leaving new full backup (and this particular incremental is deleted).
So, after the 2nd week: You now have 3 full backups (1st Monday, 1st Friday, 2nd Friday) and you have 7 incrementals (1st Tuesday, 1st wed, 1st thurs, 2nd mon, 2nd tues, 2nd wed and 2nd thursday). Overall you now have 10 restore points.
3rd monday: Incremental runs. (Thus you have hit restore point number 11). So, now the system can delelete that 1st restore point, right? Nope. That first restore point (from 1st Monday...is a full. The incrementals on 1st Tues, 1st Wed, and 1st Thursday NEED the 1st Monday full to be of any use. Therefore the system cannot delete the 1st Monday yet..as that would also delete 1st Tues, 1st Wed, and 1st Thursday. Thus leaving only 7 restore points (1st Friday, 2nd mon, 2nd tues, 2nd wed, 2nd thurs, 2nd friday, 3rd monday).
3rd tuesday: incremental runs
3rd wednesday: incremental runs
3rd thursday: incremental runs
3rd friday: Incremental first, then sythentic runs, leaving new full backup (and this particular incremental is deleted).
It's at this point, with 4 full backups, and 11 incrementals..comprising 15 retore points that anything can be deleted. After the 3rd friday backup, it can now delete 1st Monday full and 1st tues Incremental, 1st wednesday incremental and 1st thursday incremental. Once those delete, you end up with 10 restore points on disk.
Be aware, when you run your synthetic fulls you must schedule them to run on a day when you run backups or they never happen. For example, in the above scenario, if you scheduled backups to occur on M-F..but checked the box for a Saturday synthetic full, since the job won't launch on Saturday, the synethetic fulls never get created. Thus, you end up with tons of incrementals...which can never clear out as you cannot delete the very fist Full..which all subsequent incrementals depend upon.